Heretofore, there has been a lithography step as one step in a process for producing a semiconductor device or a liquid crystal display panel. In the lithography step, a resist film formed on a substrate is subjected to exposure treatment via a photomask (which may be one so-called “reticle”), so that a pattern formed on the photomask is transferred to the resist film. If foreign material is deposited on the pattern of the photomask, a pattern defect will result. Therefore, in order to prevent deposition of foreign material, a pellicle is mounted on the photomask.
Usually, a pellicle has such a structure that a transparent thin membrane (hereinafter referred to as a “pellicle membrane”) to transmit light with a wavelength of an exposure light source, is bonded to a frame via an adhesive, and is installed so that a certain distance is maintained between the pellicle membrane and the mask surface of the photomask. By this distance, even if foreign material is deposited on the pellicle membrane, it will be out of focus and will present no influence on image formation.
Heretofore, as an exposure light source to be used at the time of producing a semiconductor device or a liquid crystal display panel, an i-line light source (wavelength: 365 nm) has been predominant, and as a material for a pellicle membrane, a cellulose-type polymer such as nitrocellulose, cellulose acetate or cellulose propionate has been used.
In the fields of semiconductor devices and liquid crystal display panels, minimization of wiring and wiring interval has been in progress. Accordingly, a short wavelength has become to be selected for use as the wavelength of a light source to be used in the lithography step. The process using an i-line light source which used to be predominant, was intended for a minimum pattern dimension of at least 0.3 μm as the target. Whereas, in recent years, in order to process wiring for a minimum pattern dimension of less than 0.3 μm, a process has been introduced which uses, as an exposure light source, a light source with a wavelength of at most 250 nm such as a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm).
However, in a case where such light with a short wavelength is used as an exposure light source, the light resistance tends to be insufficient with the cellulose-type membrane material which used to be used as a material for a pellicle membrane.
In connection with such a problem, it has been found that an amorphous perfluoropolymer is useful as a material for a pellicle membrane, and various studies have been made (Patent Document 1). For example, Patent Document 2 discloses a pellicle membrane having a fluorinated layer formed on its surface, which is prepared by treating a membrane made of an amorphous perfluoropolymer with fluorine gas.
Patent Document 3 discloses a pellicle membrane made of a multilayer membrane of at least two types of amorphous fluoropolymers.